3. The message-passing function implementing foo needs to determine the first element of its argument list to see what message it should handle. It sends that argument list the message 'car with no arguments."

"At that point you might as well just combine the type name and the method name into one symbol and go back to square one."

Yeah, I admit it's not that great at handling the situation where a value wants to implement two different interfaces, but the interfaces have the same message name.

---

"This again.... Okay, here's an explanation by example. ^_^"

Alright, I'm starting to get it. Rather than representing types as a function that maps between keys/behaviors, you're representing it as a simple flat list. But that's quite possible to do with message passing as well:

I admit that's a tiny bit more verbose than your simple example, but it's nice because rather than exposing the type as an ordered sequence... we expose it as a table. Which means that rather than saying, "the 0th element is the value" we instead say, "the key 'value is the value", which makes it more robust if say... we decide to change it so the value is the 2nd element.

It also serves as self-documenting code. It also allows the built-in functions like `keys` to dispatch on the 'keys method... which doesn't work so well with a list. It also means we can easily write a macro to do the wrapping for us:

(def counter ()
(object counter
call () (++ self.'value)
value 0))

And if types are represented as tables... then you can use rep + sref to actually modify the internal representation... without needing to use extend at all. Unfortunately, if we went that route, then my simple examples like...

(fn (m)
(case m
'keys ...
'set ...))

...wouldn't really work. But that's okay. Implementing types as tables seems like it'd be more flexible and overall better. The only downside is that now they're carrying around an internal table... but how is that worse than carrying around an internal list?

So... would you consider the above (using a table to implement message passing) to be "more encapsulated" than your idea?

I don't know what you're getting at, but the cons cell (car . nil) and the argument list '(car) are supposed to be 'iso in my example; otherwise, why would I say it's a loop? I'm just referring to them in different ways to emphasize their different roles in the story.

I'm sure you're confused about something else in my example, but I don't know what it is. Here's a quick take at summarizing the issue anyway: Inspecting a list by sending it messages is incompatible with decoding a message by inspecting an argument list.

---

"The only downside is that now they're carrying around an internal table... but how is that worse than carrying around an internal list?"

Here's an old post of mine where I go over the pros and cons: http://arclanguage.org/item?id=12076. (Since then, I've learned to stop worrying about 'rep and 'annotate and love predicate dispatch. I put inheritance-aware multimethods in Lathe after that too, but I found they were less useful than I expected for what I was doing.)

With a predicate dispatch approach, it hardly matters whether the internal representation is a table or a list or even a function which dispatches to a complete getter/setter protocol, since I usually only interact with it in a single clump of extensions. Nevertheless, I do find this feature of list-based types pretty helpful:

(let (foo bar baz) rep.x
...)

---

"So... would you consider the above (using a table to implement
message passing) to be "more encapsulated" than your idea?"

Actually, I consider that example to be less encapsulated. Future code can come in and replace the 'call method and add new fields. That would be just fine with me.

...Do note that tables would have to be built into the core that way though. ;) It's to avoid loops again: Getting a table key by sending a message is incompatible with sending a message by getting a table key.

"Here's a quick take at summarizing the issue anyway: Inspecting a list by sending it messages is incompatible with decoding a message by inspecting an argument list."

Okay... let me see if I understand you. What you're saying is, that if a function is given an argument list (like with rest args), then it can't extract the data out of that list, by sending messages? Why not? A cons is represented as the following form:

(obj type 'cons
rep ...
car ...
cdr ...)

You retrieve those by using attr [1]:

(attr foo 'car)
(attr foo 'cdr)
...

Now, the argument list is represented as a series of conses, and each individual cons is a table. Incidentally, this is similar to how I'm implementing conses in Python, except I'm using classes rather than tables. So suppose you have the following:

(= args '(car))

Which is the same thing as this:

(= args (obj type 'cons
car 'car
cdr nil))
(= args.'rep args)

You can extract the car with (attr args 'car), and the cdr with (attr args 'cdr). All conses would be represented this way, including built-in types. So in the following function...

(fn args ...)

...the variable args would also be represented that way, so calling (attr args 'car) inside the function should work fine. You did mention a recursive loop earlier:

"If 'car is implemented as a function and you call it and 'car tries to get its argument using 'car, you're in trouble."

Which would be represented as follows...

(def car args
(car args)
...)

...which is obviously a naive infinite loop. But that is solved by using attr:

(def car args
(attr args 'car)
...)

So I fail to see what that (infinite loop) has to do with message passing. What have I misunderstood?

---

"Nevertheless, I do find this feature of list-based types pretty helpful:"

Hm... that does seem pretty useful/neat/nifty! Not sure if it offsets the advantages of a table, though.

Neat! Great summary. For the record, I'm leaning toward the second form, with type being an attribute on the table. Then `type` can just be a convenience function (or defined for backwards compatibility).

"no way for different types' x fields to be accessed using the same code without doing something like standardizing the field order"

Also, I found this amusing: "Hmm, that gives me an idea. Maybe what I miss most of all is the ability to tag a new datatype so that an existing utility can understand it"

Which is precisely what message passing does. But as you say, you seem to have moved onto other... different directions.

---

"...Do note that tables would have to be built into the core that way though. ;) It's to avoid loops again: Getting a table key by sending a message is incompatible with sending a message by getting a table key."

No worse than having fns in the core. :P Especially if I figure out a way to define fn using message passing. Then... tables would be primitives... and fn would be layered on top...?

---

* [1]: Why `attr`? Why not just use (foo 'car)? Because when a table has a 'call key, that will be used when calling it in functional position. In this case, it didn't have a 'call key, but in actual Arc code, conses would have a 'call key, which means that calling (foo 'car) wouldn't actually retrieve the 'car key... it would instead be equivalent to ((foo 'call) 'car). Thus, `attr` is necessary to distinguish between calling a table, and retrieving a key of the table.

"What you're saying is, that if a function is given an argument list (like with rest args), then it can't extract the data out of that list, by sending messages? Why not?"

Because that just sends arguments to another function, and that function tries to extract them, so it sends arguments to another function, and so on. At no point does any function succeed in determining what its first argument is.

"Why `attr`?"

FWIW, I believe I understand what 'attr is for. ^_^ It passes direct messages to a value without passing a "call" message. It just doesn't solve the paradox.

"But that is solved by using attr"

But how does the 'attr function extract its first argument?

---

"Not sure if it offsets the advantages of a table, though."

Me neither. ^_^ To even the field a bit:

Clojure has its own kinda wordy form of table destructuring. An Arc macro can do better than that when it's only trying to do non-recursive destructuring.

(w/keys a b c rep.x ; gets the 'a, 'b, and 'c keys
...)

I'm not sure if something this simple would help all the time, since I use alternate names for things pretty often (e.g. when dealing with two things of the same type), but it covers the more common cases.

"['the ability to tag a new datatype so that an existing utility can understand it'] is precisely what message passing does."

It's what 'extend does too. The tagging is just done in a meta way. The way I see things now, the meaning of a value is only the op-understands-value relationships it supports (duck typing?), so extending an op is a valid way to change (tag) the meaning of a value.

---

"Especially if I figure out a way to define fn using message passing. Then... tables would be primitives... and fn would be layered on top...?"

I think you gotta encode behavior in some type or other. I see two ways to avoid functions: You could have tables hold s-expressions and interpret them, or you could have tables encode the behavior themselves. The latter would make tables objects, and they'd be indistinguishable (on a high level) from message-passing closures.

"Because that just sends arguments to another function, and that function tries to extract them,"

Hm... well, I still don't get what you're saying, but I'll try implementing it and see if it's a problem or not.

---

"But how does the 'attr function extract its first argument?"

In a worst case scenario, it can be a built-in and use Python magic to avoid the paradox. Having one built-in (attr) is still waaaay better than having a ton of other built-ins (car, cons, cdr, maptable, etc.)

---

"Clojure has its own kinda wordy form of table destructuring. An Arc macro can do better than that when it's only trying to do non-recursive destructuring."

That's all well and good for high-level coding, but somebody's going to have to determine what the low-level data type will be. I'm going with a table, in py-arc, thanks to our discussions.

---

"It's what 'extend does too. The tagging is just done in a meta way. The way I see things now, the meaning of a value is only the op-understands-value relationships it supports (duck typing?), so extending an op is a valid way to change (tag) the meaning of a value."

Ah, but the key difference is: where is the operator stored? Right now we're only using global functions as operators, but we could also store them as functions in a table. This is better in some situations... and extend is better in other situations.

So yeah, I'm leaning toward duck typing as well, and actually getting rid of the `type` function completely. My idea is that the "type" of something is based solely on it's prototype: `table` would be both a prototype and a constructor of tables (at the same time). You can then create new prototypes that inherit from `table`, and voila, they're tables.

This is sorta-kinda like how JavaScript does it, except JavaScript actually screwed up prototypes, so we can do a lot better in Arc. For starters, in JavaScript you can access the prototype, but you can't change it. That's definitely gonna change: in Arc, you should be able to dynamically change the prototype.

Something's a table, but want it to be a cons? Just change it's 'prototype attribute and bam, it now inherits from cons. Anyways, then the `isa` function would test for inheritance, rather than type. So rather than saying (isa foo 'table) you'd say (isa foo table), which would mean, "does foo inherit from table?" [2].

As far as I can see, the only downside is that isa would now be O(n) rather than O(1), but the prototype chain should be small enough that it won't make a big difference in ordinary programs.

---

"I think you gotta encode behavior in some type or other. I see two ways to avoid functions: You could have tables hold s-expressions and interpret them, or you could have tables encode the behavior themselves. The latter would make tables objects, and they'd be indistinguishable (on a high level) from message-passing closures."

I'm kinda leaning toward the second approach. Even in a worst case scenario, having two built-in primitives that can't be created in Arc (tables and fns) is still better than the current situation.